In mobile telephony communication systems, nodes such as radio base stations are today arranged for MIMO (Multiple Input Multiple Output) communication and with adaptive beam forming. A node typical comprises multiple radio branches and multiple antenna columns that support beamforming and higher order MIMO. For example, an 8 TRX (transmit and receive) system with four dual polarized antenna columns may be used for 8T8R (eight transmit and 8 receive branches) 3-sector applications.
In a normal scenario for LTE (Long Term Evolution), for each sector, the node transmits cell specific reference signals such as CRS:s (cell specific reference signals) with relatively broad antenna beams, covering the sector, where each user terminal uses the received CRS:s to determine which beamformed antenna beam that is most suitable for downlink communication, from the node to the user terminal. The user terminal transmits this information to the node, which uses this information for creating such an antenna beam for downlink communication.
In uplink communication, from a user terminal to the node, beamforming is also applied, and the node is then arranged to alter beams in order to maximize the received signal quality.
However, some user terminals, further referred to as legacy user terminals, are not equipped for handling communication of this kind. One reason can be that the user terminal only features two receivers and therefore only supports two layer MIMO communication. Another reason can be that the user terminal does not support the required number of reference signals. Therefore, the node should support new user terminals as well as legacy user terminals, where the latter for example only support up to two reference signals and two layers on downlink. Support for both legacy user terminals and new user terminals is important in migration and mixed mode scenarios.
One solution to generate support for legacy user terminals that only support one or two layer MIMO communication is to map the applicable number of cell-specific reference signals as efficiently as possible to an equal number of beam ports. Efficiently can mean e.g. with good Tx power utilization or with good spatial characteristics for the applicable coverage area. The drawback of such a solution is that the performance will be similar to the performance of a sector with 2 transmit branches. It is thus desired to provide means for using the available resources as efficiently as possible for legacy users terminals.